Cambridge gets a white (space) wash

A consortium of Cambridge-based companies is to start testing white space radios in the UK, to see if they can coexist with each other, and everyone else.

The trials are permitted under a multi-site test licence issued by Ofcom, and will allow the likes of Microsoft, BT, Nokia, Samsung, Spectrum Bridge and BSkyB to set up radio links using TV channels that are locally empty.

The idea is to demonstrate how amazing such a technology is in the hope of getting Ofcom to approve national, and licence-free, use of the white spaces.

Microsoft et al would like us to think that using white space is a risk-free resource only limited by the regulator's imagination, and that the white space model of cognitive radios jumping between empty frequencies should be applied to the rest of the radio spectrum as quickly as possible, as shown in the company's promotional video.

But radios designed to find and use empty frequencies simply don't work, so white space proponents have been forced to use online databases of empty channels based on the location of the user. That means every white space access point will have to have GPS or similar built in, and be able to see the sky too; they'll also have to periodically check back with the database to see if things have changed.

The approach is fraught with technical difficulties, not least the way that radio propagation can change enormously depending on atmospheric conditions. There's also the matter of transmissions bleeding into neighbouring frequencies, which can be mitigated with good filters and clever radios, but not eliminated.

Large margins of error are supposed to avoid knocking out television transmissions, but avoiding interference with other white space users will be more difficult. Proponents of the technology love to compare white space radio with Wi-Fi, and Wi-Fi has certainly proved more robust than anticipated in sharing radio channels with nearby users - both users see a drop in speed, but are prepared to tolerate it.

And that's what the Cambridge trials are all about: to see if the radios can cope with interference generated by other white space users, as well as making sure the online database can provide accurate and timely information about the frequencies available.

Once that's proven then Ofcom can start wrestling with the question of who gets to run the database. In the USA the FCC has been trying to get a handle on that question for a year or so now, while the shortlisted companies continue flinging mud at each other: we can only hope the UK process won't be quite so pugnacious, assuming the trials prove successful. ®